Process for the preparation of non-oxygen precipitating Czochralski silicon wafers

ABSTRACT

The present invention relates to a process for the treatment of Czochralski single crystal silicon wafers to dissolve existing oxygen clusters and precipitates, while preventing their formation upon a subsequent oxygen precipitation heat treatment. The process comprises rapid thermal annealing the wafer to dissolve existing oxygen clusters and precipitates. The rapid thermal anneal is performed in an atmosphere capable of oxidizing the surface of the wafer thereby causing an inward flux of silicon self-interstitial atoms in order to reduce the number density of vacancies in the single crystal silicon to a value such that oxygen precipitates will not form if the wafer is subsequently subjected to an oxygen precipitation heat-treatment.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. provisional applicationSerial No. 60/098,822, filed on Sep. 2, 1998, and is a continuation U.S.application Ser. No. 09/379,383 filed on Aug. 23, 1999 now U.S. Pat. No.6,336,968.

BACKGROUND OF THE INVENTION

The present invention generally relates to the preparation ofsemiconductor material substrates, especially silicon wafers, which areused in the manufacture of electronic components. More particularly, thepresent invention relates to a process for the treatment of Czochralskisingle crystal silicon wafers to dissolve existing oxygen clusters andprecipitates, while preventing their formation upon a subsequent oxygenprecipitation heat treatment.

Single crystal silicon, which is the starting material for mostprocesses for the fabrication of semiconductor electronic components, iscommonly prepared with the so-called Czochralski process wherein asingle seed crystal is immersed into molten silicon and then grown byslow extraction. As molten silicon is contained in a quartz crucible, itis contaminated with various impurities, among which is mainly oxygen.At the temperature of the silicon molten mass, oxygen comes into thecrystal lattice until it reaches a concentration determined by thesolubility of oxygen in silicon at the temperature of the molten massand by the actual segregation coefficient of oxygen in solidifiedsilicon. Such concentrations are greater than the solubility of oxygenin solid silicon at the temperatures typical for the processes for thefabrication of electronic devices. As the crystal grows from the moltenmass and cools, therefore, the solubility of oxygen in it decreasesrapidly, whereby in the resulting slices or wafers oxygen is present insupersaturated concentrations.

During the thermal treatment cycles typically employed in thefabrication of electronic devices, oxygen precipitate nucleation centersmay form and ultimately grown into large oxygen clusters orprecipitates. The presence of such precipitates in the active deviceregion of the wafer can impair the operation of the device.Historically, to address this problem electronic device fabricationprocesses included a series of steps which were designed to producesilicon having a zone or region near the surface of the wafer which isfree of oxygen precipitates (commonly referred to as a“denuded zone” ora“precipitate free zone”). Denuded zones can be formed, for example, ina high-low-high thermal sequence such as (a) oxygen out-diffusion heattreatment at a high temperature (>1100° C.) in an inert ambient for aperiod of at least about 4 hours, (b) oxygen precipitate nucleiformation at a low temperature (600-750° C.), and (c) growth of oxygen(SiO₂) precipitates at a high temperature (1000-1150° C.). See, e.g., F.Shimura, Semiconductor Silicon Crystal Technology, Academic Press, Inc.,San Diego Calif. (1989) at pages 361-367 and the references citedtherein.

More recently, however, advanced electronic device manufacturingprocesses such as DRAM manufacturing processes have begun to minimizethe use of high temperature process steps. Although some of theseprocesses retain enough of the high temperature process steps to producea denuded zone, the tolerances on the material are too tight to renderit a commercially viable product. Other current, highly advancedelectronic device manufacturing processes contain no out-diffusion stepsat all. Because of the problems associated with oxygen precipitates inthe active device region, therefore, these electronic device fabricatorsmust use silicon wafers which are incapable of forming oxygenprecipitates anywhere in the wafer under their process conditions.

Accordingly, a process is needed by which existing oxygen clusters orprecipitates in the silicon wafer may be dissolved, prior to the devicefabrication, in such a way that future formation of oxygen precipitateswithin the wafer is prevented.

SUMMARY OF THE INVENTION

Among the objects of the invention, therefore, is the provision of aCzochralski single crystal silicon wafer, as well as the process for thepreparation thereof, in which oxygen clusters and precipitates have beendissolved; and, the provision of such a wafer which will not form oxygenprecipitates or clusters upon being subjected to an oxygen precipitationheat treatment.

Briefly, therefore, the present invention is directed to a process forheat-treating a Czochralski single crystal silicon wafer in a rapidthermal annealer to dissolve oxygen clusters, and to prevent futureprecipitate formation resulting from a subsequent thermal processingstep. The process comprises heat-treating the wafer at a temperature ofat least about 1150° C. in an atmosphere having an oxygen concentrationof at least about 1000 ppma to dissolve existing oxygen clusters andyield a wafer which is incapable of forming oxygen precipitates uponbeing subjected to an oxygen precipitation heat treatment.

The present invention is further directed to a process for heat-treatinga Czochralski single crystal silicon wafer to dissolve oxygenprecipitates or clusters, and to prevent future precipitate formationresulting from a subsequent thermal processing step. The processcomprises heat-treating the wafer in a rapid thermal annealer at atemperature of at least about 1150° C. to dissolve existing oxygenclusters or precipitates, and controlling the cooling rate of theheat-treated wafer down to a temperature of less than about 950° C. toproduce a wafer which is incapable of forming oxygen precipitates uponbeing subjected to an oxygen precipitation heat treatment.

The present invention is still further directed to a process forheat-treating a Czochralski single crystal silicon wafer to dissolveoxygen precipitates or clusters, and to prevent future precipitateformation resulting from a subsequent thermal processing step. Theprocess comprises heat-treating the wafer in a rapid thermal annealer ata temperature of at least about 1150° C. in an atmosphere to dissolveexisting oxygen clusters or precipitates. The heat-treated wafer is thencooled to a temperature between about 950 and 1150° C. at a rate inexcess of about 20° C., and then thermally annealed at a temperaturebetween about 950 and 1150° C. to produce a wafer which is incapable offorming oxygen precipitates upon being subjected to an oxygenprecipitation heat treatment.

Other objects and features of this invention will be in part apparentand in part pointed out hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention affords the means by which toobtain a single crystal silicon wafer having a reduced concentration ofoxygen precipitates or clusters, as well as other defects to which theseprecipitates are related. Additionally, the present process yields awafer which, during essentially any subsequent oxygen precipitation heattreatment (e.g., annealing the wafer at a temperature of 800° C. forfour hours and then at a temperature of 1000° C. for sixteen hours),will not form oxygen precipitates. The process of the present inventiontherefore acts to annihilate or dissolve a variety of preexistingdefects such as large oxygen clusters and certain kinds of oxygeninduced stacking fault (“OISF”) nuclei throughout the wafer. Thedissolved oxygen which remains in the wafer will not precipitate, evenif the wafer is subjected to an oxygen precipitation heat treatment.

The starting material for the process of the present invention is asingle crystal silicon wafer which has been sliced from a single crystalingot grown in accordance with conventional Czochralski crystal growingmethods. Such methods, as well as standard silicon slicing, lapping,etching, and polishing techniques are disclosed, for example, in F.Shimura, Semiconductor Silicon Crystal Technology, Academic Press, 1989,and Silicon Chemical Etching, (J. Grabmaier ed.) Springer-Verlag, N.Y.,1982 (incorporated herein by reference). The silicon wafer may bepolished or, alternatively, it may be lapped and etched but notpolished. In addition, the wafer may have vacancy or self-interstitialpoint defects as the predominant intrinsic point defect. For example,the wafer may be vacancy dominated from center to edge,self-interstitial dominated from center to edge, or it may contain acentral core of vacancy of dominated material surrounded by an axiallysymmetric ring of self-interstitial dominated material.

Czochralski-grown silicon typically has an oxygen concentration withinthe range of about 5×10¹⁷ to about 9×10¹⁷ atoms/cm³ (ASTM standardF-121-83). Because the oxygen precipitation behavior of the wafer isessentially erased by the present process (i.e., the wafer isessentially rendered non-oxygen precipitating, even if subjected to anoxygen precipitation heat treatment), the starting wafer may have anoxygen concentration falling anywhere within or even outside the rangeattainable by the Czochralski process. Depending upon the cooling rateof the single crystal silicon ingot from the temperature of the meltingpoint of silicon (about 1410° C.) through the range of about 750° C. toabout 350° C., oxygen precipitate nucleation centers may form in thesingle crystal silicon ingot from which the wafer is sliced.

The presence or absence of these nucleation centers in the startingmaterial is not critical to the present invention. Preferably, however,these centers are capable of being dissolved by the rapid thermal annealheat-treatment of the present invention.

In accordance with the process of the present invention, a singlecrystal silicon wafer is first subjected a heat treatment step in whichthe wafer is heated to an elevated temperature. Preferably, this heattreatment step is carried out in a rapid thermal annealer in which thewafer is rapidly heated to a target temperature and annealed at thattemperature for a relatively short period of time. In general, the waferis subjected to a temperature in excess of 1150° C., preferably at least1175° C., more preferably at least about 1200° C., and most preferablybetween about 1200° C. and 1275° C. The wafer will generally bemaintained at this temperature for at least one second, typically for atleast several seconds (e.g., at least 3), preferably for several tens ofseconds (e.g., 20, 30, 40, or 50 seconds) and, depending upon thepre-existing defects, for a period which may range up to about 60seconds (which is near the limit for commercially available rapidthermal annealers).

The rapid thermal anneal may be carried out in any of a number ofcommercially available rapid thermal annealing (“RTA”) furnaces in whichwafers are individually heated by banks of high power lamps. RTAfurnaces are capable of rapidly heating a silicon wafer, e.g., they arecapable of heating a wafer from room temperature to 1200° C. in a fewseconds. One such commercially available RTA furnace is the model 610furnace available from AG Associates (Mountain View, Calif.).

Heat-treating the wafer at a temperature in excess of 1150° C. willcause the dissolution of a variety of pre-existing oxygen clusters andOISF nuclei. In addition, it will increase the number density of crystallattice vacancies in the wafer.

Information obtained to date suggests that certain oxygen-relateddefects, such as ring oxidation induced stacking faults (OISF), are hightemperature nucleated oxygen agglomerates catalyzed by the presence of ahigh concentration of vacancies. Furthermore, in high vacancy regions,oxygen clustering is believed to occur rapidly at elevated temperatures,as opposed to regions of low vacancy concentration where behavior ismore similar to regions in which oxygen precipitate nucleation centersare lacking. Because oxygen precipitation behavior is influenced byvacancy concentration, therefore, the number of density of vacancies inthe heat-treated wafer is controlled in the process of the presentinvention to avoid oxygen precipitation in a subsequent oxygenprecipitation heat treatment.

In a first embodiment of the process of the present invention, thevacancy concentration in the heat-treated wafers is controlled, at leastin part, by controlling the atmosphere in which the heat-treatment iscarried out. Experimental evidence obtained to date suggests that thepresence of a significant amount of oxygen suppresses the vacancyconcentration in the heat-treated wafer. Without being held to anyparticular theory, it is believed that the rapid thermal annealingtreatment in the presence of oxygen results in the oxidation of thesilicon surface and, as a result, acts to create an inward flux ofsilicon self-interstitials. This inward flux of self-interstitials hasthe effect of gradually altering the vacancy concentration profile bycausing recombinations to occur, beginning at the surface and thenmoving inward.

Regardless of the mechanism, the rapid thermal annealing step is carriedout in the presence of an oxygen-containing atmosphere in the firstembodiment of the process of the present invention; that is, the annealis carried out in an atmosphere containing oxygen gas (O₂), water vapor,or an oxygen-containing compound gas which is capable of oxidizing anexposed silicon surface. The atmosphere may thus consist entirely ofoxygen or oxygen compound gas, or it may additionally comprise anon-oxidizing gas such as argon. It is to be noted, however, that whenthe atmosphere is not entirely oxygen, preferably the atmosphere willcontain a partial pressure of oxygen of at least about 0.001 atmospheres(atm.), or 1,000 parts per million atomic (ppma). More preferably, thepartial pressure of oxygen in the atmosphere will be at least about0.002 atm. (2,000 ppma), still more preferably 0.005 atm. (5,000 ppma),and still more preferably 0.01 atm. (10,000 ppma).

Intrinsic point defects (vacancies and silicon self-interstitials) arecapable of diffusing through single crystal silicon with the rate ofdiffusion being temperature dependant. The concentration profile ofintrinsic point defects, therefore, is a function of the diffusivity ofthe intrinsic point defects and the recombination rate as a function oftemperature. For example, the intrinsic point defects are relativelymobile at temperatures in the vicinity of the temperature at which thewafer is annealed in the rapid thermal annealing step, whereas they areessentially immobile for any commercially practical time period attemperatures of as much as 700° C. Experimental evidence obtainedto-date suggests that the effective diffusion rate of vacancies slowsconsiderably at temperatures less than about 700° C. and perhaps asgreat as 800° C., 900° C., or even 1,000° C., the vacancies can beconsidered to be immobile for any commercially practical time period.

In a second embodiment of the present invention, therefore, theconcentration of vacancies in the heat-treated wafer is controlled, atleast in part, by controlling the cooling rate of the wafer through thetemperature range at which vacancies are relatively mobile. As thetemperature of the wafer is decreased through this range oftemperatures, the vacancies diffuse to the wafer surface and areannihilated, thus leading to a change in the vacancy concentrationprofile with the extent of change depending upon the length of time thewafer is maintained at a temperature within this range and the magnitudeof the temperature; in general, greater temperatures and longerdiffusion times lead to increased diffusion. In general, the averagecooling rate from the annealing temperature to the temperature at whichvacancies are practically immobile (e.g., about 950° C.) is preferablyno more than 20° C. per second, more preferably no more than about 10°C., and still more preferably no more than about 5° C. per second.

Alternatively, the temperature of the wafer following the hightemperature anneal may be reduced quickly (e.g., at a rate greater thanabout 20° C./second) to a temperature of less than about 1150° C. butgreater than about 950° C. and held for a time which is dependent uponthe holding temperature. For example, for temperatures near 1150° C.,several seconds (e.g., at least about 2, 3, 4, 6 or more) may besufficient whereas at temperatures near 950° C. several minutes (e.g.,at least about 2, 3, 4, 6 or more) may be required to sufficientlyreduce the vacancy concentration.

Once the wafer is cooled to a temperature outside the range oftemperatures at which crystal lattice vacancies are relatively mobile inthe single crystal silicon, the cooling rate does not appear tosignificantly influence the precipitating characteristics of the waferand thus, does not appear to be narrowly critical.

Conveniently, the cooling step may be carried out in the same atmospherein which the heating step is carried out. Suitable atmospheres include,for example, nitriding atmospheres (that is, atmospheres containingnitrogen gas (N₂) or a nitrogen-containing compound gas, such asammonia, which is capable of nitriding an exposed silicon surface);oxidizing (oxygen-containing) atmospheres; non-oxidizing, non-nitridingatmospheres (such as argon, helium, neon, carbon dioxide), andcombinations thereof.

While the rapid thermal treatments employed in this process may resultin the out-diffusion of a small amount of oxygen from the surface of thefront and back surfaces of the wafer, the resulting heat-treated waferhas a substantially uniform interstitial oxygen concentration as afunction of distance from the silicon surface. For example, aheat-treated wafer will have a substantially uniform concentration ofinterstitial oxygen from the center of the wafer to regions of the waferwhich are within about 15 microns of the silicon surface, morepreferably from the center of the silicon to regions of the wafer whichare within about 10 microns of the silicon surface, even more preferablyfrom the enter of the silicon to regions of the wafer which are withinabout 5 microns of the silicon surface, and most preferably from thecenter of the silicon to regions of the wafer which are within about 3microns of the silicon surface. In this context, a substantially uniformoxygen concentration shall mean a variance in the oxygen concentrationof no more than about 50%, preferably no more than about 20%, and mostpreferably no more than about 10%.

In view of the above, it will be seen that the several objects of theinvention are achieved. As various changes could be made in the abovecompositions and processes without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription be interpreted as illustrative and not in a limiting sense.

I claim:
 1. A process for heat-treating a Czochralski single crystalsilicon wafer to dissolve pre-existing oxygen precipitates, the processcomprising heat-treating the wafer in a rapid thermal annealer at atemperature of at least about 1150° C. in an atmosphere having an oxygenconcentration of at least about 1000 ppma to control the crystal latticevacancy concentration to produce a wafer in which the formation ofoxygen precipitates in a subsequent oxygen precipitation heat treatmentis prevented and wherein the rapid thermally annealed wafer comprises aregion having a substantially uniform concentration of oxygeninterstitial atoms which extends from the center of the wafer to adistance of no greater than 15 microns from the surface of the wafer. 2.The process of claim 1 wherein the region having a substantially uniformconcentration of oxygen interstitial atoms extends from the center ofthe wafer to a distance of no greater than 10 microns from the surfaceof the wafer.
 3. The process of claim 1 wherein the region having asubstantially uniform concentration of oxygen interstitial atoms extendsfrom the center of the wafer to a distance of no greater than 5 micronsfrom the surface of the wafer.
 4. The process of claim 1 wherein theregion having a substantially uniform concentration of oxygeninterstitial atoms extends from the center of the wafer to a distance ofno greater than 3 microns from the surface of the wafer.
 5. The processof any one of claims 1-4 wherein the concentration of oxygeninterstitial atoms in the substantially uniform region has a variance ofno more than about 50%.
 6. The process of any one of claims 1-4 whereinthe concentration of oxygen interstitial atoms in the substantiallyuniform region has a variance of no more than about 20%.
 7. The processof any one of claims 1-4 wherein the concentration of oxygeninterstitial atoms in the substantially uniform region has a variance ofno more than about 10%.
 8. A process for heat-treating a Czochralskisingle crystal silicon wafer to dissolve pre-existing oxygenprecipitates, the process comprising heat-treating the wafer in a rapidthermal annealer at a temperature of at least about 1150° C. andcontrolling the rate of cooling from the maximum temperature achievedduring the heat-treatment through a temperature range in which vacanciesare relatively mobile for a time period sufficient to reduce the numberdensity of crystal lattice vacancies in the cooled wafer prior tocooling the wafer below the temperature range in which vacancies arerelatively mobile, to produce a wafer in which the number density ofvacancies in the single crystal silicon has been reduced to a value suchthat oxygen precipitates will not form in the heat-treated wafer uponsubjecting the wafer to an oxygen precipitation heat-treatment andwherein the thermally annealed wafer comprises a region having asubstantially uniform concentration of oxygen interstitial atoms whichextends from the center of the wafer to a distance of no greater than 15microns from the surface of the wafer.
 9. The process of claim 8 whereinthe region having a substantially uniform concentration of oxygeninterstitial atoms extends from the center of the wafer to a distance ofno greater than 10 microns from the surface of the wafer.
 10. Theprocess of claim 8 wherein the region having a substantially uniformconcentration of oxygen interstitial atoms extends from the center ofthe wafer to a distance of no greater than 5 microns from the surface ofthe wafer.
 11. The process of claim 8 wherein the region having asubstantially uniform concentration of oxygen interstitial atoms extendsfrom the center of the wafer to a distance of no greater than 3 micronsfrom the surface of the wafer.
 12. The process of any one of claims 8-11wherein the concentration of oxygen interstitial atoms in thesubstantially uniform region has a variance of no more than about 50%.13. The process of any one of claims 8-11 wherein the concentration ofoxygen interstitial atoms in the substantially uniform region has avariance of no more than about 20%.
 14. The process of any one of claims8-11 wherein the concentration of oxygen interstitial atoms in thesubstantially uniform region has a variance of no more than about 10%.15. A process for heat-treating a Czochralski single crystal siliconwafer to dissolve pre-existing oxygen clusters and to prevent futureprecipitate formation resulting from an oxygen precipitation heattreatment, the process comprising: heat-treating the wafer at atemperature of at least about 1150° C. in a rapid thermal annealer todissolve pre-existing oxygen clusters; cooling the heat-treated wafer toa temperature between about 950 and 1150° C. at a rate in excess ofabout 20° C.; thermally annealing the cooled wafer at a temperaturebetween about 950 and 1150° C. for a time period sufficient to reducethe number density of crystal lattice vacancies in the cooled waferprior to cooling the wafer below a temperature of about 950 EC, toproduce a wafer in which the formation of oxygen precipitates in asubsequent oxygen precipitation heat treatment is prevented and whereinthe rapid thermally annealed wafer comprises a region having asubstantially uniform concentration of oxygen interstitial atoms whichextends from the center of the wafer to a distance of no greater than 15microns from the surface of the wafer.
 16. The process of claim 15wherein the region having a substantially uniform concentration ofoxygen interstitial atoms extends from the center of the wafer to adistance of no greater than 10 microns from the surface of the wafer.17. The process of claim 15 wherein the region having a substantiallyuniform concentration of oxygen interstitial atoms extends from thecenter of the wafer to a distance of no greater than 5 microns from thesurface of the wafer.
 18. The process of claim 15 wherein the regionhaving a substantially uniform concentration of oxygen interstitialatoms extends from the center of the wafer to a distance of no greaterthan 3 microns from the surface of the wafer.
 19. The process of any oneof claims 15-18 wherein the concentration of oxygen interstitial atomsin the substantially uniform region has a variance of no more than about50%.
 20. The process of any one of claims 15-18 wherein theconcentration of oxygen interstitial atoms in the substantially uniformregion has a variance of no more than about 20%.
 21. The process of anyone of claims 15-18 wherein the concentration of oxygen interstitialatoms in the substantially uniform region has a variance of no more thanabout 10%.